The present invention addresses a long standing obstacle to optimized visual clarity in high speed digital continuous and drop-on-demand ink jet printing where output is created by a plurality of primary ink droplets emitted during an energizing sequence to ink emitting nozzles of print cartridges disposed in an ink jet print engine. Such primary droplets are typically accompanied by secondary and tertiary ink ("satellite") droplets which create undesirable image defects when said satellite droplets register upon a printing media at locations other than a desired primary mark location. A primary droplet is emitted under precise electronic control typically as a result of a energy pulse received by an ink emitting nozzle to thus impart a preselected trajectory and time of flight to the primary droplet. When a one of a plurality of nozzles are energized a primary ink droplet accelerates toward a preselected location on a printing media, typically trailed by one or more satellite droplets. Typically, such satellite droplets form at the tail of the primary droplet and may precede or trail each primary droplet in flight, although the manner of satellite droplet formation is immaterial for purposes of understanding the background of the present invention. Although in practice some satellite droplets may merge back into a primary droplet, at least a portion of said satellite droplets often register on the printing media outside a boundary of each primary mark created by each primary droplet and thus reduce edge clarity of said primary marks and thus overall text quality and color fidelity of a printed image. This phenomenon shall be referred to herein as "Overspray" which term shall include all those visual artifacts described and depicted herein, as well as other visual artifacts created when emitting droplets of any marking material to register precisely upon a print substrate, or an intermediate substrate, and said visual artifacts are attributable to satellite droplets.
In the prior art related to ink jet printing, a print head operated under precise electronic control typically opposes a portion of printing media so that an image may be printed thereon. Some representative prior art approaches to controlling or alleviating undesirable image artifacts created by satellite droplets include: adjustment to the excitation signal wave form to inhibit satellite formation by applying certain harmonic frequencies to a continuous jet stream as in U.S. Pat. No. 3,928,855; forming a modified nozzle face portion for emitting primary droplets to thereby minimize satellite formation as in U.S. Pat. No. 5,057,853; laser trimming ink droplet emission circuitry for each of a plurality of nozzle rows and/or otherwise adjusting the magnitude of an excitation signal to thus improve efficiency and consistency of primary droplet formation as in U.S. Pat. No. 5,389,956; and applying electrostatic charging to influence both primary and satellite droplets as in U.S. Pat. No. 5,489,929. On the whole these patents provide a perspective on controlling or minimizing formation of satellite droplets, in contrast to the present invention which assumes that ink jet printing typically produces an appreciable volume of satellite droplets.
In the relevant prior art, control of satellite droplets has not generally depended upon the type of digital print engine used to emit ink onto a printing media. In this respect both the prior art and the present invention apply without limitation to drum-based, reciprocating swath (or carriage-based) print engines, and flat bed-based digital print engines. In a drum-based print engine a print media attaches to a rotating drum which repeatedly passes under one or more discrete ink emitting print elements ("nozzles") mounted on a carriage articulated in the axial direction. In a reciprocating swath, or carriage-based, print engine the media is incrementally stepped over a fixed platen surface in one direction while the nozzles reciprocate across the media in a direction orthogonal to the direction the media advances. In a flat bed print engine, the printing media is typically rigidly coupled to a substantially planar surface and the nozzles are articulated in two dimensions to cover the media. In each of these types of prior art print engine mechanisms, the nozzles are spaced from and thus do not contact the printing media as the print head dispenses ink upon the print media to form an image. When such spacing is minimized, it is known that satellite formation and the resulting image defects are reduced; however, with less spacing between the nozzles and the printing media the likelihood and severity of damaging contact between the nozzles and printing media increases. Nevertheless, in all said prior art print engines an appreciable number of satellite droplets register upon the media and thus inhibit the clarity of printed output from said print engines.
In all such traditional drop-on-demand print engines, electronic control operates to impart a known trajectory profile to each said primary droplet emitted from the nozzles to accurately control primary droplets. The manner and effectiveness of imparting such a known trajectory profile to a primary droplet is not altered or modified herein, although some considerations for primary droplet placement typically include: time for a drive pulse to reach a firing chamber proximate an ink emitting nozzle, the time for ink to fill an ink firing chamber, the time for a nozzle to physically emit a primary ink droplet, a velocity of the emitted primary ink droplet, an expected time of flight of the primary ink droplet, and relative velocity of the print media and nozzles, among others. To the inventors knowledge, no prior art approach actually compensates and controls satellite droplets once they form and are otherwise likely to register in undesired locations on the printing media with either a column of relatively high velocity fluid or through implementing one or more fluid deflector members.
Thus, a need exists in the art of drop-on-demand ink jet and continuous ink jet printing to compensate for satellite droplets formed incidentally during formation of primary ink droplets in order to improve the quality and the visual clarity of text, graphics, and color appearing on print media. Further, a need exists in the art for a method and apparatus to reduce the visual impact of said satellite droplets whether or not additional satellite droplet reduction measures are undertaken.